In all cases where glass panels are cut for applications in architectural, automotive, consumer electronics, to mention a few areas, there will be edges, which will very likely require attention. There are as many different methods to cut and separate glass as there are edge shapes. Glass can be cut mechanically (CNC machining, abrasive waterjet, scribing and breaking, etc), using electro-magnetic radiation (lasers, electrical discharges, gyrotron, etc) and many other methods. The more traditional and common methods (scribe and break or CNC machining) create edges that are populated with different types and sizes of defects. It is also common to find that the edges are not perfectly perpendicular to the surfaces. In order to eliminate the defects and give the edges a more even surface with improved strength, they are usually ground. The grinding process involves abrasive removal of edge material that can give it the desired finishing and also shape its form (bull nosed, chamfered, pencil shape, etc). In order to allow the grinding and polishing steps, it is necessary to cut parts that are larger than the final desired dimensions.
While it is well known and understood that eliminating defects will increase edge strength, there is not an agreement on the impact that shape has on edge strength. The confusion occurs mainly because it is well known that shape helps to increase damage resistance to impact and handling of the edges. The fact is that edge shape really does not determine edge strength as defined by resistance to flexural (or bending) forces, but the defects size and distribution do have a great impact. However, a shaped edge does help to improve impact resistance by creating smaller cross section and containing defects. For example, an edge with a straight face that is perpendicular to both surfaces accumulates stress at these right angled corners that will chip and break when it is impacted by another object. Because of the accumulated stress, the size of defects can be pretty big, which will diminish the strength of that edge considerably. On the other hand, due to its smoother shape, a rounded “bull-nosed” shaped edge will have lower accumulated stress and smaller cross section which helps to reduce the size and penetration of defects into the volume of the edge. Therefore, after an impact, a shaped edge should have higher “bending” strength than a flat edge.
For the reasons discussed above, it is often desirable to have the edges shaped, as opposed to flat and perpendicular to the surfaces. One important aspect of these mechanical cutting and edge shaping methods is the degree of maintenance of the machines. Both for cutting and grinding, old and worn down cutting heads or grinding rolls can produce damage which can significantly affect the strength of the edges, even if the naked eye cannot be see the differences. Other issues with mechanical cutting and grinding methods is that they are very labor intensive and require many grinding and polishing steps until the final desired finish, which generate a lot of debris and require cleaning steps to avoid introduction of damages to the surfaces.
From process development and cost perspectives there are many opportunities for improvement in cutting and chamfering edges of glass substrates. It is of great interest to have a faster, cleaner, cheaper, more repeatable and more reliable method of creating shaped edges than what is currently practiced in the market today. Among several alternative technologies, laser and other thermal sources have been tried and demonstrated to create shaped edges.
In general, ablative laser techniques tend to be slow due to the low material removal rate and they also generate a lot of debris and heat affected zones that lead to residual stress and micro-cracks. For the same reason, melting and reshaping of the edges are also plagued with a lot of deformation and accumulated thermal stress that can peel that processed area. Finally, for the thermal peeling or crack propagating techniques, one of the main issues encountered is that the peeling is not continuous.
Subsurface damage, or the small microcracks and material modification caused by any cutting process, is a concern for the edge strength of glass or other brittle materials. Mechanical and ablative laser processes are particularly problematic with regard to subsurface damage. Edges cut with these processes typically require a lot of post-cut grinding and polish to remove the subsurface damage layer, thereby increasing edge strength to performance level required for applications such as in consumer electronics.